[Richard Crowley]

wrote ...
"Richard Crowley" wrote...
It seems quite likely that the "normaling" contacts (the ones which
connect the speakers when the headphone jack is withdrawn) are
dirty, which causes the kind if intermittent operation you describe.

You could try acquiring some contact cleaner in an aerosol (spray)
form and squirt it into the jack, then insert and withdraw a plug to
exercise (and ~clean) the contacts. Better would be to gain access
inside the equipment and spray the contacts directly.

Not exactly. Modern receivers like the Onkyo have sensing switches which,
when the headphones are plugged in, communicate to the microprocessor that
the speakers should be turned OFF and (generally)the multiple channels
should be downmixed to stereo. My guess is one or more bad solder
connections at or very near the headphone jack, or the headphone jack
itself could have sustained some minor damage from usage.

One would think that if that were the case the OP would have heard
significant clicking or even buzzing from the speaker relays, etc.